1. Technical Field
The present invention relates, generally, to semiconductor manufacturing processes and, more particularly, to wirebonding on copper metal pads during the packaging of semiconductor chips and the use of formic acid as a reducing agent during copper wirebonding.
2. Background Art and Technical Problems
In the past, semiconductor chips were typically manufactured with a top layer of aluminum metal to provide electrical interconnections among the devices (such as transistors) formed on (or in) the semiconductor chip. The metal layer would include pads which provided points where wires could be bonded to the chip to provide electrical connections to external circuits, as well as power and ground connections. Thus, suitable technology was developed for bonding wires to aluminum metal pads.
Copper now offers certain advantages as a metal layer for semiconductor manufacturing. However, problems have arisen when attempts have been made to bond wires to copper metal pads. A very short time after the copper metal layer is deposited on a semiconductor chip, a thin layer of copper oxidation forms on the surface of the metal. For purposes of this discussion, "copper oxidation" is used generically to refer to various species of insulative copper compounds that form in reaction to atmosphere or other gasses to which the copper surface is typically exposed, including but not necessarily limited to, CuO, Cu.sub.2 O.sub.3, CuSO.sub.4, to obtain a good electrical connection by wire bonding to the copper pad.
In the past, a wire would be vibrated at high frequencies to break up the layer of aluminum oxide that would typically form on aluminum metal pads. However, such techniques prove to be ineffective when used during copper wire bonding. Vibrations are effective when used to break up aluminum oxide, because aluminum oxide tends to be brittle. Copper oxidation does not have the same characteristics.
Attempts to clean the surface of copper pads to permit wire bonding to be performed have also been ineffective. The copper oxidation layer grows back too quickly. Thus, even if the copper surface can be cleaned, the copper oxidation layer will usually grow back before wire bonding can be achieved.
Efforts to achieve wire bonding to copper metalization have included sputtering a layer of aluminum over the copper metal. Other alternatives have included sputtering or otherwise depositing a layer of nichol-gold, or a layer of nichol-lead-gold. While such techniques may result in some success at wire bonding, each of these is such a high cost solution that it has proven to be less than satisfactory in practice. A lower cost solution is needed.
Therefore, with the development of semiconductor chip designs employing copper metalization, a cost effective method for effectively wire bonding to copper pads is needed.